kasli_generic: support external reference on masters

artiq/gateware/targets/kasli_generic.py

@@ -162,6 +162,10 @@ class GenericMaster(MasterBase):
             rtio_clk_freq=description.get("rtio_frequency", 125e6),
             enable_sata=description.get("enable_sata_drtio", False),
             **kwargs)
+        if "ext_ref_frequency" in description:
+            self.config["SI5324_EXT_REF"] = None
+            self.config["EXT_REF_FREQUENCY"] = "{:.1f}".format(
+                description["ext_ref_frequency"]/1e6)
         if hw_rev == "v1.0":
             # EEM clock fan-out from Si5324, not MMCX
             self.comb += self.platform.request("clk_sel").eq(1)